1. Field of the Invention
This invention relates to a lens barrel of a camera such as a video camera, and particularly to a lens barrel of a camera having a motor for controlling axial movement of a lens for focusing and zooming.
2. Description of the Background Art
A video camera has a lens barrel containing a plurality of lenses such as a zoom lens, a focusing lens and the like. Referring to FIG. 3, a conventional lens barrel 50 includes a cylindrical front barrel 51 and a cylindrical rear barrel 52 fixedly connected to the front barrel 51. The front barrel 51 has front and rear jaws 51a and 51b which extend radially inwardly from respective front and rear ends thereof. The front jaw 51a retains a focusing lens group 53 (hereinafter referred to as the focusing lens) on an inner periphery thereof. The rear jaw 51b retains a compensator lens group 54 (hereinafter referred to as the compensator lens) on an inner periphery thereof. The rear barrel 52 has a rear jaw 52b which extends radially inwardly from periphery of a rear end thereof.
A pair of guide shafts 55, 55 are supported between the front and rear jaws 51a, 51b of the front barrel 51 in parallel to an optical axis C. Further, another pair of guide shafts 56, 56 are supported between the rear jaw 51b of the front barrel 51 and the rear jaw 52b of the rear barrel 52 in parallel to the optical axis C.
The guide shafts 55, 55 pass through a first movable lens holder ring 58, so that the lens holder ring 58 is slidably supported thereon. The lens holder ring 58 retains a variator lens group 57 (hereinafter referred to as the variator lens) which fulfills a variable zoom function. The guide shafts 56, 56 pass through a second movable lens holder ring 60 so that the lens holder ring 60 is slidably supported thereon. The lens holder ring 60 retains a master lens group 59 (hereinafter referred to as the master lens) which fulfills focusing, compensating and image-forming functions.
The first movable lens holder ring 58 is provided with a radial pin 61 on an outer periphery thereof. The pin 61 is fitted into a cam groove 62a extending spirally along at least an inner face of a cam drum 62 which is supported between the front and rear jaws 51a, 51b of the front barrel 51. The cam drum 62 is connected through a pin 64 to a zoom ring 63, which is concentrically disposed outside of the front barrel 51 and rotates around the front barrel 51. When the zoom ring 63 rotates, the associated cam drum 62 rotates inside of the front barrel 51 in a clockwise or counterclockwise direction. The rotating movement of the cam drum 62 causes the pin 61 engaging the spiral cam groove 62a to move to and fro in an axial direction of the guide shafts 55, 55. Accordingly, the first movable lens holder ring 58 moves to and fro along the guide shafts 55, 55. To this end, the variator lens 57, retained by the first movable lens holder ring 58, moves forward or backward along the optical axis C.
The second movable lens holder ring 60 is threadedly engaged with a rotating shaft of a motor 65 which extends forwardly from a housing body of the motor 65 through the rear jaw 52b of the rear barrel 52. The forward portion of the rotating shaft is formed as a screw 66 which serves for engagement with the second movable lens holder ring 60. When the motor 65 is actuated, the screw 66 starts to rotate. According to rotation of the screw 66, the second movable lens holder ring 60 moves forward or backward along the guide shafts 56, 56 parallel to the optical axis C. To this end, the master lens 59 retained by the lens holder ring 60 moves to and fro in a direction of the optical axis. In FIG. 3, reference numeral 67 represents a speed reducer of the motor 65.
As mentioned above and as shown in FIG. 3, the conventional lens barrel 50 needs a screw 66 for moving the master lens 59 in addition to the two pairs of guide shafts 55, 55 and 56, 56 which are used for moving the variator lens 57 and the master lens 59, respectively. This leads to the use of a large number of parts, a complicated structure and large a size of the lens barrel so that manufacturing costs for the lens barrel are high.
Further, since the conventional lens barrel 50 includes front and rear barrels 51, 52 in which the guide shafts 55 and 56 are supported, respectively, there exists a possible risk that the focusing, variator, compensator and master lenses 53, 57, 54 and 59 may be decentered or inclined relative to the optical axis C. Upon assembling the rear barrel 52 onto the front barrel 51, careful assembly must be performed such that a reference axis for the rear barrel 52 is strictly aligned with that for the front barrel 51. However, this alignment procedure eventually suffers from the large number of parts employed therein so that an effective and accurate assembling operation becomes difficult.
FIG. 4 depicts a conventional motor mounting for the lens barrel for control of focusing and zooming. As illustrated in FIG. 4, reference numerals 70, 71 represent front and rear walls of the lens barrel, respectively. A sliding bearing 73 is provided on the front wall 70 while a drive motor 80 is mounted on the rear wall 71. Namely, upper and lower parts of a front side plate 81 of the motor 80 is fixed on the rear wall 71 by means of screws 75, 75, respectively. The front side plate 81 is provided, at the center thereof, with a sliding bearing 86. A rear side plate 87 of the motor 80 is provided, at the center thereof, with a sliding bearing 89. The motor 80 has a rotating shaft 82, which extends to the front wall 70 and is rotatably supported on the sliding bearings 73, 86 and 89. The rotating shaft 82 is provided with a screw 82a on a front side thereof. The screw 82a extends through and threadedly engages with a movable lens holder ring 76 which retains a variator lens (not shown) for zooming. The motor 80 has a stator yoke 83, a driving coil 84 contained in the stator yoke 83, and a magnetic rotor 85 which is attached around the rotating shaft 82. When the motor 80 is activated, the driving coil 84 is electrically energized so that the rotating shaft 82 and the screw 82a start to rotate. To this end, the movable lens holder ring 76 moves to and fro along the screw 82a in a direction of the optical axis.
However, such a conventional motor mounting for the lens barrel has a disadvantage that the screw 82a of the rotating shaft 82 suffers from excessive load due to the motor mounting. Namely, since the motor 80 is fixedly mounted on the rear wall 71 of the lens barrel by the screws 75, an excessive load is applied on the screw 82a when the bearings 73, 86, 89 are decentered relative to the rotating shaft 82.
In order to eliminate the excessive load L applied onto the screw 82a there has been provided another conventional motor mounting as illustrated in FIG. 5, in which like reference remarks are employed to indicate like parts of the motor mounting described above, and therefore detailed explanations thereof will be omitted hereinafter. Referring to FIG. 5, front and rear walls 70, 71 of the lens barrel are integrally connected to each other through a reinforcing member 72 which extends parallel to a screw 82a of a rotating shaft 82. The screw 82a is rotatably supported between the front and rear walls 70, 71 on bearings 73, 74 which are provided in the front and rear walls 70, 71, respectively. A tip end of the rotating shaft 82 contacts a side plate 88 formed of a leaf spring whereby the rotating shaft 82 is urged in the axial forward direction.
However, the conventional motor mounting as illustrated in FIG. 5 has a disadvantage that the reinforcing member 72 spanning the front and rear walls 70, 71 must be accomodated in the lens barrel, so that the lens barrel as a whole is greater in size and weight than that shown in FIG. 4.